日本金属学会誌

J. Japan Inst. Metals, Vol. 70, No. 1 (2006),
pp. 1-6

Solid State Synthesis of Nanocrystalline Hydroxyapatite by Non-Equilibrium P/M Processing

Hiroshi Kimura1, Kouzi Kitahara1 and Masaaki Naka2

1Department of Mechanical Engineering, School of Systems Engineering, National Defense Academy, Yokosuka 239-8686
2Joining and Welding Research Institute, Osaka University, Ibaraki 567-0047

Abstract:

The solid state P/M processing that consists of rotating-arm reaction ball milling and pulse electric discharge consolidation provide a process control methodology to develop the hydroxyapatite (HA) with unique property inherent to the nanocrystalline (nc) structure. The mechanical alloying of the powder mixture is conducive to the solid state synthesis of HACa10(PO4)6(OH)2 having the average crystallite size of 7 nm according to a reaction, 6CaHPO4·2H2O+4Ca(OH)2→nc-Ca10(PO4)6(OH)2+8H2O; this overall process is given by Johnson-Mehl-Avrami equation with the exponent of 1. The nc-Ca10(PO4)6(OH)2 powder compact can be consolidated at a full density; its rapid densification during heating is expressed by an Arrhenius-type equation of Newtonian viscous flow with the activation energy of 402 kJ·mol-1 under both pressure of 80 and 140 MPa. The average crystallite size of bulky Ca10(PO4)6(OH)2 increases from 10 to 50 nm with increasing temperature up to 1173 K. The high-speed superplastic forging is achieved having the strain rate of nearly 10-2 s-1 and the compressibility of 0.65 at a relatively low temperature of 1137 K, and characterized by the strain rate sensitivity exponent ranging from 0.7 to 1 and the activation energy of 75-113 kJ mol-1.


(Received 2005/8/2)

Keywords:

mechanical alloying, reaction ball milling, Johnson-Mehl-Avrami equation, in process nanocrystalline control densification, superplastic forging, constitutive law


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